Conventionally, an image forming apparatus employs an electrographic system, an ink jet system, a thermal recording system, a heat transfer system or the like, as its printing system. Recently, methods of reusing sheets used for a printer are proposed in order to reduce environmental burdens or to reduce carbon dioxide emissions. For example, JP-A-6-155906 proposes a system of rewriting images many times by using a thermally colorable or decolorable sheet. JP-A-11-268409 proposes a system of thermally deleting an image on a sheet that is printed with decolorable toner and thereby enabling reuse of the sheet. JP-A-10-88046 proposes a system of thermally deleting an image on a sheet that is printed with decolorable ink and thereby enabling reuse of the sheet. In any of these systems, a reused sheet gradually deteriorates in quality if the deletion of the image is repeated. The sheet with deteriorated quality may not be used as a medium for forming an image thereon.
JP-A-2001-209284 discloses a technique of printing, on a sheet, reuse information about reuse together with an image to be printed with erasable ink. In the technique disclosed in JP-A-2001-209284, an area in which printing of an image is disabled (non-printable area) is set on a sheet and reuse information is printed in the non-printable area. However, in the technique disclosed in JP-A-2001-209284, the area in which an image as a print target can be printed is reduced in order to secure the non-printable area on the sheet. Moreover, in the technique disclosed in JP-A-2001-209284, since the size of the non-printable area is changed, an image as desired by a user may not be printed on the sheet even if an image for printing is prepared in accordance with the sheet size.
JP-A-2001-209284 also discloses a technique of printing the remaining number of times of printing as reuse information. However, the remaining number of times of printing disclosed in JP-A-2001-209284 shows the value acquired by counting down from the limit number of times of reuse every time printing is carried out once. That is, in JP-A-2001-209284, the remaining number of times of printing is decided irrespective of the print rate on the sheet or the characteristics of the image forming material (ink or toner). Therefore, the remaining number of times of printing may not correspond to the actual damage to the sheet.